The present invention relates to a method for moving workpieces in a vacuum chamber to maximize throughput. More specifically, it involves methods that minimize the amount of time between wafer processing by utilizing a parallel flow of workpieces for processing in the vacuum chamber. Each workpiece undergoes the same steps of introduction to the vacuum chamber, alignment to a suitable orientation, processing, and removal from the vacuum chamber. Four wafers are at various phases of the wafer handling and processing cycle simultaneously, thus maximizing utilization of the equipment while minimizing cycle time.
Up to twenty different types of tools are employed for affecting several hundred processing steps during the processing of wafers in the manufacture of microelectronic circuits. Most of these processing steps must be performed in a vacuum chamber at pressures less than 1×10−3 torr, and each requires from about ten seconds to three minutes per wafer. Most of the processing tools operate on wafers one at a time in order to optimize control and reproducibility in a manufacturing environment.
In general, each operation on a wafer must be performed in a particular order, so that each operation must wait until completion of a preceding one, and, in turn, affects the time a wafer is available for a subsequent step. Tool productivity or throughput for vacuum processes that are relatively short, such as ion implantation, can be severely limited if the work flow to the processing location or platen is interrupted by sequential events, which may include, for example, the introduction of the wafer into the vacuum system, the orientation of a wafer in the vacuum chamber or the exchange of wafer carriers or cassettes.
It is desirable to shorten the duration of sequential events, i.e., those events that must be performed consecutively in order to increase throughput. However, the pump down times (to high vacuum) and the venting times (to atmospheric pressure) must be relatively long to reduce turbulence and ensure the wafer remains free of particles and foreign materials that could be redistributed from the load lock surfaces to the wafer.
The prior art has sought to address these concerns in a number of ways. For example, U.S. Pat. No. 5,486,080 of Sieradzki, which is incorporated herein by reference, employs two wafer transport robots to move wafers from two load locks past a process station. Both robots alternately transport each wafer from the cassette at one of the load locks along a path to an orientation position, through the process station, and back to the cassette until all the wafers in the cassette are processed. Pumpdown or venting of the other (second) load lock with another cassette holding multiple wafers occurs while the wafers in the cassette at the first load lock are processed. After processing the wafers from the first load lock, the first load lock is closed and vented while the second load lock is opened and the robot then transport the wafers from the second load lock through the process station. This procedure adequately achieves high throughputs for a cassette loaded batch of wafers (200 mm wafers), but does not address the requirements resulting from the use of 300 mm wafers.
With the continuing trend toward smaller and faster electronic devices, the use of cassettes to hold and transport wafers is now burdensome. For example, 300 mm wafers are transported in Front-Opening Unified Pods (FOUPs), which keep the wafers in an ultra-clean environment. The FOUPs interface to dedicated modules on process equipment, which automatically open their doors while an atmospheric robot removes and replaces wafers as required. These FOUPs are not intended to be loaded into vacuum, whereas cassettes used to transport 200 mm wafers may be directly placed in load locks and brought to vacuum. As such, a system optimized for 300 mm wafers has different requirements from earlier systems.